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This story originally featured on The Drive.

Vehicles are getting more technologically complex, physically larger, and wildly heavy—I’m looking at you, GMC Hummer. Some of that is due to increased safety standards for rollover risk, offset hits, and the need for high-strength steel in a vehicle’s crash structure, but safety is not the only cause. There’s also the abundance of electrified offerings that added weight due to dense battery systems.

No matter the reasoning behind this trend of scaling up, the result is the same: crappy handling. Weight and size make for horrendously slow directional changes. The old adage of the difficulty in course correcting a ship comes to mind. Consumers, however, still demand sports-car handling from their 6,000-pound Rolls-Royce Cullinans, which is why four-wheel steering has quickly become one of the mechanical mainstays of the automotive world. 

The term four-wheel steering has a few variations, such as all-wheel steering and rear-wheel steering (as in Tesla’s latest Cybertruck announcement). With so many terms and even brand names floating out there, it can get mighty confusing for the average person thumbing through the latest Porsche brochure.

To better shed light on rear-wheel steering, which was invented in 1893, The Drive’s Guides & Gear section put together the ultimate explainer on four-wheel steering systems, how they work, who makes them, and how they affect your ride’s performance. Let’s get rollin’. 

What is four-wheel steering?

Rear-wheel steering is a mechanism that, when the steering wheel is turned, actuates the rear wheels’ angles and either turns them in tandem with the front wheels or in the opposite direction to better increase a vehicle’s low- or high-speed agility.

We’ll get into the different types of rear-wheel steering systems in a moment, but the general premise of these systems is using a set of hydraulic or electric actuators to change the toe of the rear wheels.

Toe refers to the angle a wheel is pointed in or out from the car, hence the terms “toe in” and “toe out.” Toe can also be described as negative and positive. In most situations, toe refers to a car’s front wheels and, depending on whether both wheels are pointed in or out, it affects the car’s high-speed stability (toe in) or low-speed agility (toe out).

In rear-wheel steering, the vehicle’s system changes both rear wheel’s toe in tandem, which means the rear wheels move right or left in unison so they’re pointed in the same direction. This is the complete opposite of how toe works on a vehicle’s front wheels.

Rear-wheel steering is a mechanism that, when the steering wheel is turned, actuates the rear wheels’ angles and either turns them in tandem with the front wheels or in the opposite direction to better increase a vehicle’s low- or high-speed agility.

We’ll get into the different types of rear-wheel steering systems in a moment, but the general premise of these systems is using a set of hydraulic or electric actuators to change the toe of the rear wheels.

Toe refers to the angle a wheel is pointed in or out from the car, hence the terms “toe in” and “toe out.” Toe can also be described as negative and positive. In most situations, toe refers to a car’s front wheels and, depending on whether both wheels are pointed in or out, it affects the car’s high-speed stability (toe in) or low-speed agility (toe out).

In rear-wheel steering, the vehicle’s system changes both rear wheel’s toe in tandem, which means the rear wheels move right or left in unison so they’re pointed in the same direction. This is the complete opposite of how toe works on a vehicle’s front wheels.

The different types of four-wheel steering

Different manufacturers have different solutions to rear-wheel steering, although they all serve the same purposes. Some rely on mechanical-only systems, such as Nissan’s old HICAS (High-Capacity Actively Controlled Steering) system, while most utilize electro-mechanical versions, such as Audi’s Dynamic All-Wheel Steering system, Porsche’s Rear-Axle Steering, and the Active Kinematics Control (AKC) systems supplied by ZF to manufacturers such as Ferrari and Cadillac.

Again, these two systems work virtually identically by changing the rear wheels’ toe in or out, but they differ on what actuates them. In a mechanical system like Nissan’s HICAS, which hasn’t been used in Nissans since the R34-generation GT-R or in Infinitis since the G37, hydraulics powered by the power-steering pump were used to actuate the wheels. Speed sensors would then be used to determine which way the rear wheels would turn and how much. Honda’s Prelude Si 4WS had a similar system.

Electromechanical four-wheel steering systems are now far more common and more advanced than those earlier versions. Controlled by the car’s ECU and a number of sensors along its drivetrain, today’s four-wheel steering systems offer far more precise wheel placement, angles, and capabilities than their predecessors.

Although the basic result is the same, manufacturers also differ in how much steering angle the rear wheels allow, ranging from 1 to 15 degrees. 

Read the rest of the explainer and see some helpful diagrams over at The Drive.

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